It is known that field-effect transistors with an MIS (metal/insulator/semiconductor) structure have a metal electrode called a grid applied to one surface of a layer of insulating material whose other surface is covered with a layer of semiconductor material disposed between a current input and an output electrode called the source and drain, respectively. Application of a potential to the grid creates an electrical field that causes charges to appear in the semiconductor in the vicinity of the insulating layer. Thus, in the semiconductor zone located between the source and the drain, a bridge is created which can be made conducting and which is generally called a "channel." When the grid is energized and the channel becomes conducting, the transistor is called an "enrichment type," because its principal current increases when a potential is applied to the grid. In other systems, the channel conducts in the absence of voltage applied to the grid, and it is application of a potential to the grid that blocks the channel current. This is then called an "impoverishment type" transistor.
Classical MIS structure field-effect transistors, commonly called MISFET, are generally made on a silicon substrate strongly doped to make it conduct. A metal layer is deposited on one surface of the substrate so that the grid voltage can be applied. An insulating silica layer is made to grow on the other surface of the substrate. To this silica layer a semiconducting layer and the two metal contacts constituting the source and drain are applied. The source and drain may be in contact with the insulating layer or disposed above the semiconductor layer.
It has recently been proposed that thin-layer field-effect transistors be made in which the semiconductor is a thin layer of an organic polymer such as polyacetylene; see for example European Patent Application 0,298,628. In the transistors of this European patent application, the insulating layer is made with classical mineral insulators (silica, silicon nitride) but it is also planned to use polymers with good insulating properties for this purpose, without however any specific examples of such an insulator and such an embodiment being given.
It is also known that replacement of semiconductor organic polymers by oligomers with a given molecular weight improves the mobility of the charge carriers. Thus, a TFT type thin-layer transistor has recently been described in which the semiconductor is alpha-sexithienyl deposited on an insulator constituted by silica. Such a transistor, under the field effect, has evidenced distinctly improved carrier mobility relative to the mobility observed with a comparable device whose semiconductor is a polymer: polymethylthiophene. Indeed, in the case of alpha-sexithienyl, the mobility of the carriers under the field effect reaches 0.84.times.10.sup.-3 cm.sup.-2. V.sup.-1. s.sup.-1. This value is 10 to 100 times that obtained with comparable transistors using organic polymers such as poly(3-methylthiophene) and polyacetylene as semiconductors; see for example G. Horowitz, et al., Solid State Communications, Vol. 72 No. 4, pp. 381-384 (1989).